1. Field of the Invention
The present invention relates to the art of separating materials according to their electromagnetic properties and is specifically concerned with the processes and apparatus for electrodynamic separation and classification of nonmagnetic free-flowing materials according to their electrical conductivity and density.
The invention is particularly useful in beneficiation of auriferous samples in geological practice, in processing auriferous concentrates at concentration plants, or in separation of secondary nonferrous metals at the nonferrous metallurgy enterprises processing industrial wastes. The invention may also be employed for extracting nonferrous metals from solid domestic wastes with subsequent separation of said metals from one another.
2. Description of the Prior Art
It is common knowledge that eddy currents are induced in electrically conducting particles exposed to a variable magnetic field. Interaction of the eddy currents with a variable nonuniform magnetic field produces electromagnetic forces directed towards less intense regions of the magnetic field and causes the electrically conducting particles to move progressively from a region of higher intensity of the magnetic field to a point of a lesser intensity. The magnitude of the forces depends on the specific electrical conductance of the particles, their size and shape, as well as on the magnitude of intensity, degree of nonuniformity, and frequency of the magnetic field.
The above effect is employed in electrodynamic methods for separation of nonmagnetic metalliferous free-flowing materials.
A method and an apparatus for separating nonmagnetic materials whose particles differ in the specific electrical conductance and density are disclosed in U.S. Pat. No. 1,829,565. The separation apparatus comprises a solenoid coil connected to a high-frequency alternating current source. In separating by this method, a flow of freely falling particles being separated is fed close to the coil end. The variable magnetic field of the coil induces eddy currents in the electrically conducting particles moving close to the coil end. Interaction of the variable nonuniform magnetic field of the coil with the eddy currents in the particles produces electromagnetic forces acting on the electrically conducting particles in the direction of decrease in intensity of the coil magnetic field. Force interaction between the eddy currents and the nonuniform magnetic field of the coil results in deflecting the electrically conducting particles from the direction of their free fall, whereas the direction of free fall of electrically nonconducting particles remains unaffected. The flow of particles being separated is thus divided into at least two flows.
It is well known, however, that the intensity of the magnetic field is maximum at the point of intersection of the coil's symmetry axes and declines towards the coil ends. Inasmuch as the particle separation zone is in this case close to the coil end, it is reasonable to say that separation is effected at the magnetic field periphery, i.e., in a region where its intensity is low. Hence, the electromagnetic forces acting upon the particles being separated are weak and the separation quality is poor. Increase in the magnetic field intensity by increasing the current through the coil raises the power consumption and causes an excessive heating of the coil.
Another prior-art method of and apparatus for electromagnetic separation of nonmagnetic free-flowing materials are disclosed in French Pat. No. 2,116,430.
According to this method, a flow of particles of the material being separated is fed to the periphery of a variable magnetic field. This apparatus, called an electrodynamic separator, comprises an electromagnet having an excitation winding connected to an alternating current source and a closed magnetic core with an air gap defined by the electromagnet pole pieces.
The flow of particles of the material being separated is fed into the separation zone, i.e., into the region of variable nonuniform magnetic field, by a drum or a belt conveyor provided for this purpose. In the first case, the electromagnet is installed inside the drum so that the pole pieces are as close to the drum inner surface as possible; in the second case, the conveyor belt carrying the material being separated is arranged above the electromagnet poles.
In both of the above cases, the separation process occurs in the region of weak magnetic field, since the material being separated is spaced apart from the pole pieces (in the first case by the drum wall, and in the second by the conveyor belt).
Owing to the presence of a ferromagnetic magnetic core, the above method and apparatus partly reduce the power consumed for electromagnetic separation; the presence of the magnetic core reduces the current drawn by the excitation winding at the same magnetic field intensity in the separation zone, or in the region through which the flow of particles being separated passes. However, the magnetic field of the electromagnets in the above cases is utilized inefficiently as the major portion of magnetic flux closes in the magnetic air gap and only an insignificant portion of magnetic flux closes through the region where the flow of particles being separated passes, i.e., the magnetic field intensity in the separation zone is much lower than it is in the magnetic air gap between the electromagnet poles.
The presence of the magnetic core with a closed magnetic system in the above apparatus reduces the power consumption, but the throughput rate and separation quality are inadequate due to the fact that the magnetic field intensity in the separation zone is much lower than it is in the gap between the poles. Moreover, an unjustified power consumption is observed.